The recent advances in microfabrication technologies have developed higher integration of so-called micromachines, LSIs and the like. Thus, it is desired to improve accuracy in measuring a three-dimensional shape of a microstructure having a complex step shape. As an apparatus for measuring the three-dimensional shape of the microstructure having the complex step shape, a white light interference measuring apparatus using a light source (white light source) having a broadband spectrum has been proposed.
In the white light interference measuring apparatus, a return light reflected from an object to be measured after having reached the object from the white light source and a return light reflected from a reference mirror after having reached the reference mirror from the white light source are made to interfere with each other. Thus, white light fringes are found. Thereafter, an optical path length from the white light source to the object to be measured is scanned by the use of a piezoelectric element or the like to detect a position where the amplitude of the white light fringes is set the maximum, in other words, a position where the optical path length from the white light source to the object to be measured is set equal to an optical path length from the white light source to the reference mirror. Accordingly, a three-dimensional shape of the object to be measured is measured.
Moreover, such a white light interference measuring apparatus is used not only for measurement of the three-dimensional shape of the microstructure but also for measurement of a film thickness of a dielectric multilayer film, for structural analysis of a continuum (diffuser) such as eyegrounds and skin, for example, and the like.
In such a white light interference measuring apparatus, it is important to accurately specify the position where the amplitude of the white light fringes is set the maximum. Methods heretofore proposed as a method for identifying the position where the amplitude of the white light fringes is set the maximum are classified broadly into the following two methods.
One is a method using Fourier transform to find a position where the envelope of the amplitude of interference fringes is set the maximum in a signal region. The other is a method using a phase gradient of a Fourier spectrum in a spectral region to calculate a position where the amplitude of white light fringes is set the maximum. In a communication theory, it is generally known that phase information is more reliable to cope with nonlinear characteristics of a detector or quantization noise than signal amplitude information. This means that, in the case of white light fringes, the use of not amplitude information but phase information can improve accuracy of identifying the position where the amplitude of the white light fringes is set the maximum.
Moreover, in Patent Document 1, described is a method for identifying a 0th-order fringe position in the following manner. Specifically, in the method, on the basis of interference fringe data on an object to be measured, which are respectively obtained by interference lights having a plurality of wavelengths, extracted is phase information for each of the interference fringe data corresponding to the interference lights having the respective wavelengths. Thereafter, the phase information is used to prepare a sinusoidal function for each interference fringe data. Subsequently, a phase of the sinusoidal function is determined for each of the interference fringe data so as to set the interference fringe data to have the maximum value at a position of predetermined coordinates within a measurement region for the object to be measured.
Patent Document 1: Japanese Patent Application Laid-open Publication No. 2000-266508